International projectionist (Jan 1959-Dec 1960)

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Volume 34 JUNE 1959 Number 6 By ROBERT A. MITCHELL Screen Light Requirements in Modern Projection I: Screen Illumination and the Heat Problem E (VEN THOUGH screen-brightness standards have remained unchanged (1030 foot-lamberts) , the demands made upon projection lamps by drive-in theatres and widescreen processes have become increasingly severe. For the most part, tne arclamps wnich were powerful enough for small-screen indoor projection are now hopelessly inadequate. Not only are present-day screens bigger but more light must be forced through smaller film apertures when standard non-anamorphic prints are shown. Specifically, the non-anamorphic widescreen aperture of 1/1.85 aspect ratio (0.446" x 0.825") has only 74.33% the area of the standard 1/1.375 aperture (0.600" x 0.825"). The widescreen aperture thus passes only 0.74 to 0.80 as much light as a standard aperture, the exact amount depending on the uniformity of illumination across the aperture. [If the utilized central zone of the "spot" thrown upon the aperture by the arclamp has a high degree of uniformity, certainly no less than about 75% of "normal" radiation gets through the widescreen aperture.] Greater Light Requirements Yet, because of the wider screens in use, this 75% of normal illumination must be spread out by short-focus lenses over a screen surface having 1.345 times "normal" screen area if producing power of the arclamps by a factor of 1/0.75 x 1.345 =1.8 times to obtain the same incident light (footcandles) and the same picture brightness (foot-lamberts) which prevailed with the old format. Apparent screen brightness may be increased with the same intensity of incident light by using directional-type screen surfaces, but we do not believe that arclamp power should be deliberately reduced just because a slightly brighter screen is going to be installed. The anamorphic ( CinemaScope ) process poses a lighting problem which is different in theory but the same in practice. The CinemaScope aperture of 1/2.35 aspect ratio (0.715" x 0.839") passes about 1.21 times the light passed by a standard aperture. This does not mean 21%. more screen lumens with CinemaScope, however, for the anamorphic attachment absorbs and wastes from 10% to 20% the original picture height is main or more of the light, depending on tained, as it is in most theatres. This which type of attachment it is. (There means that we must increase the light are three in wide use.) Although the resultant screen-lumen value with a CinemaScope aperture and anamorphic attachment is ordinarily very nearly the same as the standardaperture lumen value, the light must be spread out (at an aspect ratio of 1/2.35) over a screen area a little more than 1.7 times "normal" area. Because this factor is very close to that for 1/1.85 non-anamorphic projection (1.8), we may state as a practical rule that lamp power must be increased approximately 1.75 times to obtain with widescreen projection (any commonly used 35-mm process) the same picture brightness that prevailed with the old standard format, with the type of screen surface and its vertical dimension remaining the same. DriveIn Screens Too Dim This handy and easily remembered rule of "1.75 times standard-aperture lumens" is extremely important when the old conventional-screen brightness level was at a barely acceptable minimum (9 or 10 foot-lamberts). It signifies the difference between 10,000 and 17,500 lumens, or between 25,000 and 43,750 lumens. Drive-in screens are so large (often exceeding 100 feet in width) that projection technologists have been forced to adopt for them a minimumbrightness standard of only 4 footlamberts. Despite this compromise with screen size, however, surveys re INTERNATIONAL PROJECTIONIST • JUNE 1959